One widely used method for forming a pattern such as metallic circuitry on a substrate in the manufacture of printed circuit boards in integrated circuits is to imagewise form a pattern of photoresist material over those areas of the substrate or over a metal-coated substrate to be shielded from metal deposition or metal removal. The photoresist layer is normally formed of a polymeric, organic material that is substantially unaffected by the metal deposition or metal removal process and, accordingly, protects the underlying areas.
Another method for forming such a pattern in a substrate is referred to as the "lift off" method that involves imagewise forming a pattern of photoresist material on substrate leaving exposed those areas where the pattern of, for instance, the metallic circuitry is to be provided. Then depositing over the entire surface the metal followed by removing the remaining photoresist which in turn removes the metal located above it, while leaving remaining the desired metallic circuitry.
The pattern in the above methods is formed by imagewise exposing the photoresist material to electromagnetic radiation through a photographic image, such as a glass master, by photolithographic techniques. The electromagnetic radiation is usually x-ray, U.V. radiation, or electron beam radiation.
A number of photoresist materials are well known and are capable of forming a desired masking pattern. However, providing compositions which exhibit acceptable imaging (resolution and acuity) characteristics to be suitable as an image or pattern mask, especially for integrated circuits, along with the ability to tailor the composition to providing either a positive or negative resist, is quite unusual.
Also, providing such compositions with the ability to achieve a readily strippable composition that is also etch resistant is even that much more difficult.
For photoresist technology, epoxy materials have significant advantages over other materials because of their excellent adhesion and structural integrity after curing. These advantages have meant that epoxies could be formulated into high resolution and high aspect ratio photoresists. However, stripping off these materials after the photoresist has served its purpose can be a problem because, as highly crosslinked network polymers, they are insoluble.
Similarly, a lift-off layer for fabricating microcircuits is only useful if it can be easily dissolved to remove unwanted overlaying material from the microcircuit. Thermoset epoxies cannot be used for this purpose despite some attractive properties because as crosslinked network polymers, they cannot be easily dissolved. An easily removed, crosslinked epoxy would be attractive as a lift-off layer because it could be applied with little or no solvent and cured in place, resulting in little shrinkage and excellent planarization.